The pills are basically just really addictive, to the point that, after having one of them just once, the withdrawal kills 100% of the time after 3 days of not having a pill (or of not having at least one of the pills if they've been given both). They do have their stated effects though... alongside the side effect of making them lose some of their sanity and sense of self each dose, becoming mindless slaughter machines after three or four pills, and still dying after five or six. IIRC